1. Field of the Invention
The present invention relates generally to the deposition of hard coatings onto large-area substrates by ion-assisted sputter deposition, and, more particularly, to improved method and apparatus related to magnetron sputtering deposition with simultaneous high flux ion bombardment of the substrate. Also more particularly, the present invention is directed to the low-temperature (.ltoreq.450.degree. C.), plasma-ion-assisted, high-rate deposition of hard thin films onto large-area, three-dimensional, irregularly shaped objects without the requirement of substrate manipulation.
2. Description of Related Art
To form hard coatings at low substrate temperature, i.e., equal to or less than about 450.degree. C., a thin film method must satisfy several requirements simultaneously. For example, the case of depositing a hard coating of titanium nitride onto a substrate is now considered. The first requirement is that energetic (several eV) atoms, such as titanium, must be delivered to a substrate surface uniformly. The use of an energetic atom aids in improving tile atom mobility on the substrate to nucleate and grow the deposited film at low temperature. Additional requirements are that the substrate surface must be heated uniformly and that the deposited film must be simultaneously subjected to a high-flux (&gt;1 mA/cm.sup.2) of energetic (&gt;50 eV) ion (e.g., argon ion) bombardment sufficient to influence the film microstructure to form a hard coating.
With ion bombardment (&gt;1 mA/cm.sup.2, &gt;50 eV), the hardness of the coating can be increased by a factor of 3 times or more, compared to the hardness of a coating without ion bombardment. For titanium nitride, this corresponds to an increase in the Vickers hardness from 500 Kg/mm.sup.2 to 1,500 Kg/mm.sup.2 in the presence of ion bombardment.
Sputtering is a technique known to produce energetic (about 5 eV) titanium atoms for deposition of thin films onto surfaces. Thermally evaporated titanium has an energy of about 0.1 to 0.2 eV, which is generally not enough to provide sufficient energy for low temperature film growth. Ionized titanium has an energy of about 50 to 100 eV, which is higher than sputtering. However, for depositing thin films uniformly onto sharp corners, ionized atoms deposit non-uniformly because of the non-uniform electric fields, whereas energetic atoms do not respond to electric fields. Therefore, energetic sputtered atoms are more suitable for uniform deposition at edges, corners, and flat surfaces and at lower temperatures.
For heating of the substrate, electron bombardment or ion bombardment are techniques known to be applicable prior to thin-film deposition. Also, ion beams or plasmas are techniques known to provide fluxes of ions for bombarding substrates during thin-film deposition and which are sufficient for influencing the film microstructure.
A variety of techniques are used to deposit thin films, such as titanium nitride. These include reactive evaporation, arc evaporation, sputter-CVD (chemical vapor deposition), reactive magnetron sputtering, unbalanced magnetron sputtering, and hollow cathode magnetron sputtering.
The leading process using reactive evaporation (RE) is disclosed and claimed in U.S. Pat. No. 4,197,175, "Method and Apparatus for Evaporating Materials in a Vacuum Coating Plant" (called the RE process herein). In this process, electrons are extracted from a separate arc discharge chamber into the process chamber. The electrons are used to ionize the Ar and N.sub.2 gas atoms in the process chamber as well as to evaporate titanium metal from a crucible and subsequently ionize the evaporated titanium atoms. The substrate samples are first biased positively to attract electrons from the arc discharge chamber for heating to the substrate. The samples are then biased negatively to attract Ar.sup.+ ions which first sputter-clean the substrate surface and then bombard the film during deposition to influence the film microstructure as well as maintain the substrate temperature during the deposition.
There are a number of disadvantages of the foregoing process. The evaporation of titanium and the ion bombardment of the sample cannot be independently controlled since the electron beam is used both for Ar.sup.+ and N.sub.2.sup.+ plasma production, and for evaporation of titanium atoms from crucible and their subsequent ionization. The titanium is evaporated practically from a point source, which limits the large-scale capability of the process. Complicated sample fixturing and sample manipulation involving rotation of the sample is required. Even with a high degree of ionization of titanium from the evaporator, a high percentage of the titanium deposited onto the surface of the substrate is comprised of neutral titanium atoms. These atoms have the characteristic energy of thermally evaporated atoms: about 0.1 to 0.2 eV. Because of this low energy, a high substrate temperature, about 500.degree.0 C., is required to deposit hard titanium nitride films.
To summarize, in the RE process, film growth is conducted by argon-ion bombardment of the substrate surface simultaneously with the film deposition, sufficient to influence the film microstructure. However, a high percentage (&gt;30%) of the deposited titanium is in the form of non-energetic (0.1 to 0.2 eV) titanium atoms requiring high (500.degree. C.) substrate temperature for hard, thin-film growth. Also, the argon-ion plasma-production process is coupled to the titanium-atom evaporation and ionization process, which does not allow independent control of each process. Furthermore, since titanium atoms are supplied by a point evaporation source, complicated sample manipulation is required and the capability of processing large samples is very limited.
In sputter-CVD, such as disclosed and claimed in U.S. Pat. No. 4,992,153, "Sputter-CVD Process for at Least Partially Coating a Workpiece", a magnetic field-assisted sputtering device is used to generate sputtered atoms which then deposit onto a substrate to grow a hard film in the presence of a separate plasma, which may be argon. The substrate is rotated and the sputter target is held fixed in position. While sputtering is suitable for large-scale deposition, there are several limitations of the sputter-CVD process. Principal among them is the fact that the substrate either electrically floats or is biased positive with respect to the plasma.
When the substrate is allowed to electrically float, ion bombardment from the plasma is insufficient to influence the film microstructure. Any ion bombardment that does occur cannot be controlled without changing the plasma conditions.
When the substrate is biased positive with respect to the plasma, electrons from the plasma bombard the substrate instead of ions. Therefore, in this operating mode, ions do not influence the film microstructure. Instead, electron heating is relied upon for accomplishing this.
Other limitations of the sputter-CVD process include the use of only one sputter target which severely restricts the large-scale processing capability. To process large-scale, three-dimensional, irregularly-shaped substrates, substrate manipulation is required which is cumbersome.
To summarize, in the sputter-CVD process, film growth occurs via energetic (several eV) sputtered titanium atoms; however, there is no simultaneous ion-bombardment of the substrate that is sufficient to influence the film microstructure which is crucial to growing hard, thin films at low substrate temperature. Instead, electron bombardment heating is relied upon to accomplish this. The sputter-CVD process is also restrictive in its capability to process large-scale, three-dimensional objects.
Magnetron sputtering is a process that is well-developed for large-scale deposition of thin films and is described by P. Martin and R. Netterfield, "Ion Assisted Di-electric and Optical Coatings", in Handbook of Ion Beam Processing Technology: Principles, Deposition, Film Modification and Synthesis, edited by J. Cuomo et al, Noyes Publications, New Jersey (1989). The sputtered atoms reach the substrate surface with a typical energy of a few eV (.apprxeq.5 eV), which makes the deposition temperature of the film between 50.degree. to 100.degree. C. lower compared with the reactive evaporation technique described earlier. However, it is well-known that for low temperature (less than about 450.degree. C.) growth of thin films, high-flux (&gt;1 mA/cm.sup.2), energetic (&gt;50 eV) ion-bombardment is necessary for forming hard, wear-resistant TiN films. Furthermore, in the conventional magnetron sputtering system, the plasma is confined very near the sputtering target. Biasing of the substrate surface does not allow enough ions to be collected from the magnetron plasma to form high quality, hard thin films.
To circumvent this limitation, two techniques have been developed: the unbalanced magnetron, described by B. Window and N. Sawides, "Unbalanced DC Magnetrons as Sources of High Ion Fluxes", Journal of Vacuum Science and Technology A, Vol. 4, No. 3, pp. 453-457 (May-June 1986); and the hollow cathode magnetron target, described by J. J. Cuomo and S. M. Rossnagel, "Hollow-Cathode-Enhanced Magnetron Sputtering", Journal of Vacuum Science and Technology A, Vol. 4, No. 3, pp. 393-396 (May-June 1986).
In the unbalanced magnetron, the magnetic field at the magnetron target is adjusted and controlled so that the magnetic field lines extend to the substrate surface. This increases the plasma density near the substrate surface to improve the flux rate of ions bombarding the surface. However, use of this process has limitations.
First, the only source of plasma is from the magnetron source itself. Therefore, argon-ion bombardment is coupled to the operation of the magnetron, even though the magnetic field can be varied independently to influence the plasma density. More importantly, the magnetic field configuration of the unbalanced magnetron does not allow for efficient and uniform processing of large-scale substrates that are three-dimensional in shape, or that are magnetic. For example, to treat a cast iron (magnetic material) substrate that is 3-ft.times.3-ft in size, an extremely strong magnetic field must be generated at the magnetron in order for it to extend over a large distance from the magnetron and envelop the large size of the object. The operation of the unbalanced magnetron is therefore dependent upon the size and shape of the substrate and must be tailored differently for each substrate. Furthermore, since cast iron is magnetic, this will non-uniformly terminate the magnetic field lines, extending from the magnetron, resulting in a non-uniform plasma and subsequent treatment of the substrate by plasma-ion bombardment. In this case, the substrate size, shape, and type (magnetic vs. non-magnetic) will couple to the plasma production process downstream of the magnetron. Finally, electron heating and argon-ion sputter cleaning of the substrate surface cannot be performed because the only source of plasma is from the magnetron, which necessarily produces titanium atoms once a plasma is established.
To summarize, in the unbalanced magnetron process, film growth occurs via energetic (few eV) sputtered titanium atoms and simultaneously with high-flux, energetic ion bombardment from a plasma to influence the film microstructure. However, the plasma production is coupled to the magnetic field extending from the magnetron, not allowing independent control of each parameter. In addition, the process is ill-suited for treatment of large-scale, three-dimensional objects, and for magnetic materials, as well.
In the hollow cathode magnetron technique, some of these limitations are alleviated. The technique is based on the use of a triode discharge in which an auxiliary source of electrons is coupled to the magnetron cathode. The hollow cathode is inserted into the fringe field of a planar magnetron, near the front cathode surface. The hollow cathode is started and biased sufficiently below the plasma potential of the magnetron so that several amperes of electron current can be emitted into the magnetron plasma. These electrons cause additional ionization of the magnetron plasma and allow the operating pressure of the magnetron to be lowered to the high 10.sup.-5 Torr range.
In this pressure range, the magnetron is compatible with the use of a separate broad-beam ion gun of the Kaufman type, which requires operation at relatively low pressures. However, the limitation of this version of the hollow cathode magnetron technique is that three-dimensional, irregularly shaped objects cannot be processed because of the line-of-sight directionality of the ion gun ion beam. In addition, ion current densities in excess of several mA/cm.sup.2 required to influence the film microstructure are limited using un-neutralized ion beams because of space-charge effects. Finally, it is important to realize that the hollow cathode enhances only the efficiency of the magnetron. It does not enhance the plasma density near the substrate to aid in ion bombardment of the film during deposition.
To summarize, in the hollow cathode magnetron technique, film growth occurs via energetic (few eV) sputtered titanium atoms and with simultaneous argon-ion bombardment from a separate ion-beam source to influence the film microstructure. However, the technique is ill-suited to processing of large-scale objects because of the line-of-sight restriction of the ion beam and the limitation to obtain ion current densities of several mA/cm.sup.2 over large areas.
In arc evaporation, described by D. Sanders, "Vacuum Arc-Based Processing", in Handbook of Plasma Processing Technology: Fundamentals, Etching, Deposition, and Surface Interactions, S. M. Rossnagel et al, eds., Noyes Publications, New Jersey (1990), use is made of a Ti plate that is operated at cathode potential. No external plasma is required to produce Ti atoms. Instead, a vacuum cathodic-arc is created between the Ti plate and an anode surface that forms an arc spot on the cathode. The arc spot produces an intense Ti plasma. This arc spot moves around on the Ti plate. Ti atoms, ions, and Ti clusters consisting of macroparticles of Ti atoms are evaporated from the arc spots. When deposited onto the substrate surface, the clusters can make the surface rough and leave weak spots in the coating. In the arc evaporation process, an additional restriction is that high deposition rates are limited. The reason for this is that when the arc-evaporation power increases, the titanium cluster evaporation rate increases, which severely degrades the film quality. This is in contrast to sputtering of titanium, where an increase in sputter power continuously increases the sputter rate of titanium.
Arc evaporation has a high degree of ionization of Ti. However, like the reactive evaporation process, the residual neutral Ti atoms produced during arc-evaporation dominate the film quality, requiring a high substrate temperature compared to magnetron sputtering. For high deposition rates, a low substrate temperature is difficult to maintain because of the additional substrate heating that is generated when energetic (50 to 100 eV) titanium ions deposit onto a substrate.
In arc evaporation, electron heating and Ar.sup.+ -ion sputter cleaning are not possible because there is no Ar.sup.+ ion plasma produced. In addition, sputter cleaning of the substrate surface prior to deposition of the film is not possible because the production of the vacuum arc is coupled to the production of titanium. Once an arc is struck, titanium is produced from the arc and deposited onto the substrate. No argon plasma is produced separately from this arc process.
To summarize, in the arc evaporation technique, film growth is dominated by non-energetic (0.2 eV) thermal titanium atoms despite the production of energetic (ionized) titanium ions from the arc evaporation process. Simultaneous argon-ion bombardment of the substrate to influence film microstructure is absent.
Thus, there remains a need for apparatus and a method for depositing hard, thin films at relatively low temperature and high rate onto large-scale, three-dimensional, irregularly shaped objects without the need for substrate manipulation.